重构项目结构，移除Assignment相关功能，优化Submission模块

TechHelper.Server/Services/Grade/IGradeService.cs

@@ -0,0 +1,10 @@
+using Entities.DTO;
+using SharedDATA.Api;
+using TechHelper.Services;
+
+namespace TechHelper.Services.Beta
+{
+    public interface IGradeService : IBaseService<GradeDto, Guid>
+    {
+    }
+}